The present invention relates to a method of determining the rate of dilution of the lubricating oil by the fuel of an internal combustion engine by measuring the radioactivity of a radioactive tracer introduced into the lubricating oil or the fuel, as well as a device such as a test bench for an internal combustion engine making it possible to implement this method.
For both automobile manufacturers and manufacturers of lubricating oils and/or functional additives for engine oils the importance is known of the precise knowledge of the phenomena of dilution of lubricants by the fuels within the new generation of direct injection engines, in particular those with a positive ignition or a compression ignition.
Thus, in a compression ignition engine, commonly called a diesel engine, the principle of fuel supply by direct injection within the combustion chamber is the basis for a transfer of a part of the fuel to the bottom end where it mixes with the lubricating oil.
This transfer of fuel towards the lubrication system of the engine is further accentuated for engines equipped with exhaust fume post-treatment systems, such as particle filters or catalytic converters. Indeed, in engines equipped with such post-treatment systems, additional injections of fuel can be carried out on the level of the combustion chambers at a moment when the fuel will not be burned there but sent towards the exhaust line where it will be used to regenerate the exhaust fume post-treatment system, for example for the combustion of the soot accumulated in particle filters or to modify the state of oxidation of the medium inside the catalytic system. The rate of dilution of the lubricating oil by the fuel can thus reach values of 10% by volume and more.
The introduction of fuel into the oil lubrication system has as a consequence, on the one hand, the degradation of the characteristics of the lubricant, for example a reduction of its viscosity, a dilution of the additives, and, on the other hand, an increase in the volume present within the oil pan. It results in a change in the operation of the engine which manifests itself by reduced oil pressure and abnormally high oil consumption and, in the long term, in increased wear of mechanical components, and even engine breakdown.
However, the use of exhaust fume post-treatment systems is expanding because of increasingly strict anti-pollution standards and, as a consequence, the resolution of the problem of the dilution of lubricating oil referred to above constitutes an important challenge for the automobile industry.
Automobile manufacturers must now, and in the future, perform numerous tests in developing engines exhibiting a controlled, and if possible a minimal, dilution of the lubricating oil by the fuel injected into the combustion chambers.
A certain number of techniques exist, such as packed-column or capillary gas chromatography, which make it possible to assay the quantity of fuel in oil samples taken from within an engine on a test bench. However, these analysis techniques are discontinuous, relatively complex, consume the analyzed oil sample which thus can not be reintroduced into the lubrication system and require a relatively long analysis time.